1. Field of the Invention
The present invention relates to a charging device of corona discharge type, an image forming apparatus comprising the charging device, and a method for forming a discharge electrode.
2. Description of the Related Art
Heretofore, in an image forming apparatus which employs an electrophotographic system, a charging device of corona discharge type (corona discharge device) has commonly been used for a charging device for charging a photoreceptor, a transfer device for effecting electrostatic transfer printing of a toner image to a recording paper sheet, a separating device for effecting electrostatic separation of a recording sheet, and so forth.
As a corona discharge device, there is known a corona discharge device of so-called corotron type, which comprises a shield case having an opening formed to face an object to be charged such as a photoreceptor or a recording paper sheet, and a discharge electrode disposed in the interior space of the shield case. In the corotron-type corona discharge device, upon application of a high voltage, corona discharge takes place at the discharge electrode. A stream of ions generated through the corona discharge travels toward an object to be charged so as to generate a discharge current, with the consequence that an object to be charged is brought into a charged state.
As another corona discharge device, there is known a corona discharge device of so-called scorotron type, which is constructed by adding a grid electrode to the structure of the corotron-type corona discharge device. The grid electrode is disposed between a discharge electrode and an object to be charged. In the scorotron-type corona discharge device, a voltage of predetermined level is applied to the grid electrode at the time of corona discharge, so that an object to be charged can be charged even more uniformly. However, in the corona discharge devices of corotron type and scorotron type, there is a need to pass a large quantity of discharge current to bring electric discharge into a condition of stability, which gives rise to the problem of generation of large amounts of ozone.
As a charging device other than the corona discharge device, there is known a contact-type charging device having a charging member formed of a semiconducting roller or brush. In this construction, the charging member is brought into contact with or placed proximately to face an object to be charged, and a voltage is applied between the charging member and the object to be charged, so that the object to be charged can be charged. According to the contact-type charging device, the region of electric discharge is limited to a minute gap created in the vicinity of the part of contact between an object to be charged and the charging member. Therefore, in the contact-type charging device, in contrast to the corona discharge device, the amount of discharge current can be reduced. Accordingly, the contact-type charging device is capable of reduction in the amount of ozone generation.
However, the contact-type charging device poses the following problems. The charging member is prone to abrasion and quality degradation due to the contact with an object to be charged, electrical stress, and so forth, which makes it difficult to achieve speeding-up of a charging process, as well as to impart a long operable life to the charging member. Furthermore, the charging characteristics of the contact-type charging device are likely to deteriorate due to changes in the properties of the charging member ascribable to contamination, environmental conditions, a lapse of time, and so forth.
In addition to that, a technology to achieve superimposition of multi-color images on a photoreceptor (called IOI: Image On Image) has been developed in recent times. The IOI technology affords the advantages of being less prone to displacements of images of a plurality of colors and of causing little image quality deterioration because of just one time of a transfer process being required, and is therefore excellent in production of high-quality images. However, the IOI technology is not adapted to the use of a contact-type charging device. Accordingly, the IOI technology necessitates a non-contact type charging device which exhibits high charging uniformity.
In view of such circumstances, in the design of corona discharge devices of corotron type, scorotron type, etc. attempts have been made to achieve a reduction in the amount of ozone generation, an increase in longevity, improvement in charging characteristics, and so forth.
In Japanese Unexamined Patent Publication JP-A 6-11946 (1994), there is disclosed a charging device built as a corona discharge device having a discharge electrode of serrated configuration. In the corona discharge device having, like the discharge electrode of serrated configuration, a discharge electrode having sharp-pointed projections, an electric field tends to be concentrated on the projections, and also the number of electric discharge points is reduced. Accordingly, even if the level of a voltage to be applied is relatively low, it is possible to effect corona discharge, wherefore generation of ozone can be suppressed.
However, in the corona discharge device equipped with a discharge electrode having projections, variations in the state of electric discharge are caused by abrasion of the projections, adhesion of discharge products, and so forth. This leads to unevenness in the charged potential of an object to be charged in the direction of the length of the corona discharge device, with the consequent possibility that the charging uniformity of an object to be charged will be impaired. In the event of, for example, abrasion of the projections, in order to prevent impairment of the charging uniformity, as a condition for voltage application, the level of a voltage to be applied is set to be higher than normal so that required discharge current can be generated even at the projection in a state where electric discharge is less likely to occur. However, an increase in the level of an applied voltage results in excessive electric discharge at the projection in a state where electric discharge occurs readily. This leads to occurrence of unnecessary electric discharge that does not contribute to charging of an object to be charged, with a consequent undesirable increase in the amount of ozone generation.
As a technique to overcome such a problem, in Japanese Unexamined Patent Publications JP-A 5-2314 (1993) and JP-A 8-160711 (1996), there is disclosed a technology to divide a discharge electrode of serrated configuration into pieces on a projection-by-projection basis so as to connect an electric resistor element between each of the projections and a power source. In such a structure, in the projection where the amount of discharge current is large, a drop in voltage caused by the connected electric resistor element is significant, and the applied voltage is decreased correspondingly, wherefore corona discharge is restricted. On the other hand, in the projection where the amount of discharge current is small, a drop in voltage caused by the connected electric resistor element is insignificant, and the applied voltage is increased correspondingly, wherefore corona discharge is accelerated. Thus, according to the technology presented in JP-A 5-2314 and JP-A 8-160711, variations in a stream of ions among the projections can be reduced, with the consequent improvement in charging uniformity. Moreover, since satisfactory charging uniformity can be attained even if the applied voltage is decreased to reduce the total amount of discharge current, it is possible to reduce the amount of ozone generation. It is noted that, however, the manufacturing cost will be increased because of the necessity for providing an electric resistor element in the corona discharge device.
In Japanese Unexamined Patent Publication JP-A 7-104549 (1995), there is disclosed a technology to achieve improvement in charging uniformity in a scorotron-type discharge device by reducing the aperture ratio of a grid electrode opposed to a tip end portion of a discharge electrode, viz., a discharge region, so that part of a stream of ions can be absorbed by the grid electrode. According to the technology presented in JP-A 7-104549, improvement in charging uniformity can be achieved in a simple manner with low cost. However, the negative side is that, as a stream of ions traveling toward an object to be charged is absorbed by the grid electrode, there will be a drop in the charged potential of an object to be charged correspondingly.
In Japanese Unexamined Patent Publication JP-A 11-212335 (1999), there is disclosed a charging device comprising an electric field regulation member to eliminate a ripple in charged potential, as will hereinafter be described. A paragraph [0026] of JP-A 11-212335 states that the pitch of projections of a discharge electrode should preferably be increased to achieve reduction in the amount of ozone generation and improvement in the charging uniformity of an object to be charged as well. In order to verify this suggestion, an electric discharge test was performed on each of a case under a condition where the pitch of projections of a discharge electrode is narrow and a case under a condition where the pitch of the projections thereof is wide. The measurement of a charged potential at an object to be charged has been carried out by means of an experiment system as shown in FIG. 2 that will hereinafter be described. In a corona discharge device having no grid electrode (corotron), discharge electrodes of varying projection pitch were mounted individually. Upon charging a photoreceptor, a charged potential on the surface of the photoreceptor was measured in a direction along the length of the photoreceptor.
FIGS. 8A to 8D are views showing a relationship between a projection pitch in a discharge electrode and charging uniformity. As to the condition where the projection pitch of a discharge electrode A1 is narrow as shown in FIG. 8A, as shown in FIG. 8B, irregular fluctuations were observed in the charged potential of the photoreceptor. Furthermore, as the result of observation of the projections of the discharge electrode A1 under this condition, as shown in FIG. 8A, it has been found that, among the projections, some undergo light emission A2 resulting from electric discharge, but others don't, with consequent lack of uniformity in the state of electric discharge. Thus, when the state of electric discharge is not uniform, although it is possible to attain at least practically acceptable charging uniformity by, as has already been described, increasing discharge current or by providing a narrow grid electrode as presented in JP-A 8-160711, unnecessary electric discharge has to be conducted. This leads to an increase in the amount of ozone generation as is undesirable.
On the other hand, as to the condition where the projection pitch of the discharge electrode A1 is wide as shown in FIG. 8C, as shown in FIG. 8D, a ripple took place in the charged potential. However, this is not irregular fluctuations but regular periodic fluctuations that occur at intervals substantially equivalent to the pitch distance between the projections. Moreover, as the result of observation of the state of light emission during electric discharge, as shown in FIG. 8C, it has been found that each and every projection undergoes light emission A2 resulting from electric discharge and that electric discharge takes place at each and every projection in a relatively stable condition. Further, a comparison was made between the case under the condition where the pitch is narrow and the case under the condition where the pitch is wide in respect of the amount of ozone generation. At this time, the amount of discharge current for the former case and that for the latter case were set at the same value. The result is that the case under the condition where the pitch is wide yielded a reduction in the amount of ozone generation. It is noted that, however, it was found to be difficult to eliminate the ripple occurring in the charged potential under the condition where the pitch is wide in spite of the provision of a grid electrode.
In this regard, according to the JP-A 11-212335, with the provision of an electric field regulation member between a tip end portion of the discharge electrode and another tip end portion adjacent thereto, a stream of ions coming from the projection can be deflected in the direction of the length of an object to be charged. This makes it possible to achieve improvement in charging uniformity, and further achieve both reduction in the amount of ozone generation and improvement in charging uniformity at one time.
However, even if the charging device presented in JP-A 11-212335 is adopted for use, there still remains unevenness in charged potential. This problem will be described hereinbelow.
At first, an electric discharge test was conducted with use of a conventional corona discharge device. FIGS. 9A to 9C are views showing how electric discharge is to be effected in the conventional corona discharge device. The conventional corona discharge device is a scorotron-type corona discharge device having stylus electrodes H arranged at regular intervals. Instead of a grid electrode, a counter electrode T for permitting arrival of a stream of ions generated is disposed at a location spaced a predetermined distance away from the tip end of the stylus electrode H. With this construction, an electric discharge test was conducted. At that point in time when dozens of hours have elapsed since the start of the electric discharge test, as shown in FIG. 9B, elliptical traces of demarcations of ion streams were observed on a surface of the counter electrode T. As will be understood from the demarcation traces of ion streams, as shown in FIG. 9A, a stream of ions is readily diffused in a direction perpendicular to the direction of arrangement of the stylus electrodes H. However, as shown in FIG. 9C, in the direction of arrangement of the stylus electrodes H, ion streams generated from the adjacent stylus electrodes H, respectively, are repelled by each other and are thus less likely to diffuse uniformly.
It has thus been found that, in the conventional corona discharge device such as presented in JP-A 6-11946, even in the absence of abrasion of the projections, adhesion of discharge products, and the like problem, the charged potential of an object to be charged is caused to drop at a position thereof opposed to a point midway between the adjacent projections due to the repulsion of ion streams, with a consequent deterioration in the charging uniformity of an object to be charged. It has also been found that, even in the case of designing the apparatus so that electric discharge occurs at all of the projections by setting the projection pitch to be relatively wide or by inserting an electric resistor element as in the charging device presented in JP-A 5-2314 and JP-A 8-160711, the charged potential of an object to be charged is caused to drop at a position thereof opposed to a point midway between the adjacent projections, and that the drop of the charged potential of an object to be charged becomes increasingly significant as the pitch of the projections is increased.
Next, for verification of the inability of the charging device presented in JP-A 11-212335 to resolve the above-described problem of a drop in the charged potential of an object to be charged, an electric discharge test was conducted with use of a charging device 57 as shown in FIG. 10 that is identical in structure with said charging device. FIG. 10 is a schematic diagram showing the charging device 57 as viewed from a surface of an object to be charged. The charging device 57 comprises a plurality of projections 55 and electric field regulation members 54. The electric field regulation members 54 are arranged symmetrically with respect to a straight line Z which passes through the tip end of the projection 55 and is thus perpendicular to the direction of arrangement of the projections 55.
In the charging device 57, the electric field regulation member 54 caused a stream of ions 56 generated from the projection 55 to spread all around so as to be deflected in substantially square form. In this way, the stream of ions 56 generated from the projection 55 diffused in a wider area than does a stream of ions spreading in elliptical form.
However, the streams of ions 56 generated from the adjacent projections 55, respectively, were repelled by each other, and consequently the extent of diffusion in the direction of the length of the charging device 57 was lesser than in the case where no electric field regulation member 54 is provided. Therefore, upon moving an object to be charged relative to the direction of the width of the charging device 57, then the object to be charged was inconveniently moved relatively along the demarcations of the streams of ions 56, in consequence whereof there resulted a drop of a charged potential in streak form on the object to be charged. This gave rise to deterioration in the charging uniformity of the object to be charged.
FIG. 11 is a graph showing a distribution of charged potentials at an object to be charged in the case of using the charging device 57. It will be understood from the graph that, upon charging an object to be charged by the charging device 57, in contrast to a position P1, a position P2, a position P3, a position P4, a position P5, a position P6, and a position P7 on the object to be charged that are opposed to their respective projections 55, at positions on the object to be charged near a midway point M1 between P1 and P2, a midway point M2 between P2 and P3, a midway point M3 between P3 and P4, a midway point M4 between P4 and P5, a midway point M5 between P5 and P6, and a midway point M6 between P6 and P7, respectively, viz., at each position thereon opposed to a point midway between the adjacent projections, a drop in the charged potential occurs. It has thus been found that there is still lack of uniformity in charging on an object to be charged even in the case of using the charging device 57 comprising the electric field regulation member 54.